1. Field of the Invention
The field of the present invention is methods and apparatuses for producing electrical energy from body motion, and in particular ankle motion of the human body.
2. Background
Over the past years the number of electrically powered devices carried by people has greatly increased; the additional batteries needed to power these devices have also increased. Many researchers have attempted to harvest energy from human kinetics. In one example, 2.5 Watts of energy was harvested from the knee joint with a 1.6 kg device (1.6 W/kg). Donelan, J. M., Q. Li, V. Naing, J. A. Hoffer, D. J. Weber, A. D. Kuo, “Biomechanical Energy Harvesting: Generating Electricity During Walking with Minimal User Effort, Science 8 Feb. 2008: Vol. 319. no. 5864, pp. 807-810. Other devices include a shake driven flashlight (1.3 W/kg), and a backpack with piezoelectric crystals in the straps (7.4 Watts from 38 kg load at fast walking). Rome, L C, L Flynn, E M Goldman, and T D Yoo. “Generating electricity while walking with loads.” Science, 309:1725-1727, 2005.
Other approaches have attempted to harvest energy from the heel strike. In one instance, an electrostatic generator was constructed by compressing a dielectric elastomer at the heel (0.8 Watts at 2 steps/sec). However, the durability of this device is low, on the order of 100,000 steps. Paradiso, J., and T. Starner, “Energy Scavenging for Mobile and Wireless Electronics,” Pervasive Computing, 2005, pg 18-27. This last reference also describes the production of a shoe with spring/generator system at the heel which produced only 0.25 Watts during heel strike. It also describes placing piezoelectric crystals in the insole, but only about 0.01 Watts of energy was produced.
U.S. Pat. Nos. 7,659,636 and 7,652,386, both to Donelan, describe a method for energy harvesting across the knee joint using sensors to disengage one of the mechanical coupling of the generator from the joint or to disengage the electrical generator from the load.
U.S. Pat. No. 7,645,246 to Rastegar describes a device for harvesting energy form ankle motion having a spring which is selectively engaged and disengaged during select periods of the walking gait cycle. The engagement occurs only during the range of ankle motion from which energy is to be harvested. The engagement/disengagement mechanisms are described as being both power-activated or manual, with both types achieving the purpose of not inducing resistance in the gait cycle during those periods when energy is not being harvested.
Methods for energy harvesting have also had poor results in the past due to the difficulty of attaching the generator to the joint and efficiently harvesting the electricity.